The present invention relates generally to a method for manufacturing a grid for gating a stream of charged particles.
Certain types of particle measurement instruments, such as ion mobility spectrometers, can require a gating device for turning on and off of a flowing stream of ions or other charged particles. This is accomplished by disposing a wire grid within the path of the ions; alternately energizing or de-energizing the grid then respectively traps the ions or allows them to flow.
Certain types of time of flight spectrometers, such as those described in the paper by Vlasak, P. R., et al., entitled xe2x80x9cAn interleaved comb ion deflection gate for m/z selection in time-of-flight mass spectrometery,xe2x80x9d in Review of Scientific Instruments, Vol. 67, No. 1, January 1996, pp. 68-72, also utilize a gating device.
The most common methods for accomplishing this use an interleaved comb of wires also referred to as a Bradbury-Nielson Gate. Such a gate consists of two electrically isolated sets of equally spaced wires that lie in the same plane and alternate in potential. When a zero potential is applied to the wires relative to the energy of the charged particles, the trajectory of the charged particle beam is not deflected by the gate. To deflect the beam, bias potentials of equal magnitude and opposite polarity are applied to the two sets of wires. This deflection produces two separate beams, each of whose intensity maximum makes an angle alpha with respect to the path of the un-deflected beam.
One approach to manufacturing a gating grid is disclosed in U.S. Pat. No. 4,150,319 issued to Nowak, et al. In this technique, a ring-shaped frame is fabricated from a ceramic or other suitable high temperature material. The two sets of wires are wound or laced on the frame. Each set of wires is actually a single, continuous wire strand that is laced back and forth between two concentric series of through-holes that are accurately drilled around the periphery of the frame.
Another technique for manufacturing such a gate is described in U.S. Pat. No. 5,465,480 issued to Karl, et al. In this approach, the gating grid elements are produced from a thin metal foil by cutting or etching the foil to produce the grid structure. The gird elements are connected to side electrodes in a desired pattern to produce the two sets of wires. The foil grid structure is made mechanically stable by attaching it to an insulating support member. After the then-rigid grid structure is affixed to the insulating support member, the grid elements are selectively severed from the side electrodes to form the interdigitated grid.
Yet another approach for manufacturing such a grid is described in the paper by Kimmel, J. R., et al., entitled xe2x80x9cNovel Method for the Production of Finely Spaced Bradbury-Nielson Gates,xe2x80x9d in Review of Scientific Instruments, Vol. 72, No. 12, December 2001, pp. 4354-4357. In this method, a guide is first manufactured out of a polymer block. The guide has a series of evenly spaced parallel grooves. A hole is drilled through the center of the polymer block; this hole eventually carries the ion beam. The machined polymer block is mounted on an insulated face of an H-shaped portion of a single sided, copper clad circuit board, with the grooves running from top to bottom of the H. The polymer-to-copper clad contacts are then fixed using an epoxy. Two small portions of the single sided copper clad board are fixed on the bottom side of the polymer in the region where the block extends over the center bar of the H-shaped copper frame.
A hand cranked, rotating screw is then used as a weaving instrument. In particular, a gold-plated tungsten wire runs from a spool over a directing screw and is coupled to the hand cranked screw by a belt. The loose end of the wire is then fixed such as by using an epoxy. A weight is hung from the wire between the directing screw and the spool in order to provide a constant tension on the wire.
Beginning at one side of the center hole, the hand crank is turned, which rotates the frame, drawing the thread from the spool. While watching through a microscope, an assembler feeds a first wire set through alternating grooves in the surface of the polymer and around the frame, making sure to touch both contacts on each pass. After winding the wire across the entire width of the opening, the wire is bound to both copper contacts on either side of the hole using an epoxy. A razor blade is then used to remove the segment of the wire between the two contacts on the side of the frame opposite the polymer.
Using the same procedure as for the first wire set, a second wire set is then wound through the grooves located between the wires of the first set. The ends of the wires are then cut, leaving wire only on the polymer side of the frame.
There are deficiencies with each of the prior art approaches to fabricating such grid elements. For example, the technique described in the Nowak patent relies upon the precise placement of two sets of aligned holes on either side of a ring. Since it uses a single strand of wire which is hand woven through the holes, it does not take into consideration the need to assure a constant mechanical tension among wires in the assembled grid. Unless the mechanical tension is relatively uniform across all wires of the grid, undesirable artifacts are introduced by irregular tension. For example, at elevated operating temperatures, the larger coefficient of expansion of the metal as compared to the ceramic support could also cause the wires to sag, potentially shorting them out if they are not properly pre-tensioned. Likewise, the imprecise nature of tensioning the wire by hand often leads to wires that are not uniformly parallel. Therefore, the field normal to the grid does not decay as rapidly as theoretically possible.
Additionally, for high speed applications, the phase delays resulting from propagation of the bias current along the single continuous strands from the contact point may cause the ions to experience a deflection at different times, depending upon where they happen to be in the beam path.
Furthermore, because the frame in Nowak is circular, the individual wires are of different lengths. This means that each wire then presents a different characteristic impedance to current flowing through it. This likewise introduces different effects to different ions, depending upon where they happen to be in the beam path. Thus, ions traveling the center of the beam are subjected to a different electrical force than ions traveling in the outer portion of the beam where the grid wires are shorter.
Finally, the required thickness of the support structure in Nowak limits how closely two grids can be placed with respect to each other.
Kimmel""s approach, similar to Nowak""s, weaves a single thread around a frame. It also requires the assembler to carefully feed the wire through one of the alternately spaced grooves. The individual wires in the set are then bound to the copper contacts using epoxy. The method of machining a polymer block to small tolerances of 0.005 mm for each grid wire can require relatively expensive machine tools.
Furthermore, if the single wire breaks during winding or any part of the process one must start over again, from the beginning, to restring the wire. The assembly procedure envisioned is apparently so tedious that Kimmel himself estimates that it takes approximately three hours to manufacture a single gate.
The presence of large amounts of insulating polymer surfaces near the beam path may cause substantial charging effects which could be detrimental to the operation of the gate, particularly for gating low energy electrons. Furthermore, a device formed from a polymer with epoxy bindings may not survive the high expected operating temperatures of some applications such as ion mobility spectroscopy.
The process described in the Karl patent does provide a grid having wires with uniform tension. A separate support structure for the foil-like grid element is be fabricated from tubes and the thin metal foil must then be attached to the grid structure. This geometry is apparently convenient for ion mobility spectroscopy, but does not allow slit or apertures to be spaced closely on both sides. While the rapid charging and discharging of the gate is facilitated by the bus-like structure, the xe2x80x9cearsxe2x80x9d extending beyond the gate are likely to produce strong reflections which would be detrimental for ultra high speed operation such as in electron TOF spectroscopy. Finally, the rotational symmetry of the Karl device is not convenient for accurate alignment of the grid wires with respect to apertures placed before or after the gate.
The present invention seeks to overcome these deficiencies with a design for a gating electrode and method for fabricating it as follows.
The grid is fabricated using a substrate formed of a ceramic, such as alumina. The substrate serves as a rectangular frame for a grid of uniformly spaced wires stretched across a center rectangular hole. On either side of the frame, nearest the hole, a line of contact pads are formed.
Adjacent the line of contact pads, on the outboard side thereof, are formed a pair of bus bars. The contact pads and bus bars provide a way to connect the wires into the desired two separate wire sets of alternating potential. Specifically, the pads formed on each side of the opening serve as contact points for one end of each wire. The pads are alternately and evenly spaced along each side of the opening, inboard of the bus bars. In a preferred embodiment, the pads may be spaced, for example, down each side of the center opening. The pads serve as electrically open termination points for the ends of the grid wires that are not connected to the bus bars.
The bus bars serve to interconnect wires that belong to a given wire set.
Steps are also performed for fabrication of the grid according to the invention. First, the support frame is made from an insulating substrate such as alumina. A rectangular shaped center hole is formed in the alumina or other ceramic. The support frame, which may be laser cut, for example, may be one inch by one inch with a one-half inch by one-half inch hole placed in its center.
Metal film is then deposited on the surface of both sides of the ceramic through vacuum evaporation of gold, using chrome as an adhesion layer, for example. The metal film is then patterned on the front side to form the conducting elements on either side of the hole. These conducting elements include the ground plane, left and right bus bars, and pad elements. The desired metalization pattern can be defined by a photo-resist and chemical-etch process, a lift-off process, or by using a physical mask during an evaporation. The metal on the back side remains, as deposited, to serve as a ground plane.
In the next sequence of steps, the grid wires are attached to the fabricated frame. In this process, a spool of wire is provided that will serve as grid wires. In one preferred embodiment, the wire is a 0.002-inch diameter gold wire and the spacing of adjacent wires is 0.020-inch, to achieve a transmission of approximately 90%. A tensioner is provided to place constant tension on the wire. The spool, for example, may be arranged on a mandrel, and a hanging weight attached to the end of a string wrapped around the mandrel. The weight is adjusted to tension the wire at a specific chosen value less than the yield strength of the wire.
The free end of the wire is then fixed to a wire clamp so that it may be precisely located with respect to the tip of a parallel gap welder. The frame is then moved so that the first pad on the left hand side of the frame is located under the tip. At this point, the wire is bonded to the center of the pad. The parallel gap welder provides a relatively immediate bond of the wire to the pad. The assembler can then pull the free end of the wire to break it free from the bond, or wait until later to cut off the free ends of all of the wires.
The wire is then bonded to the bus bar on the right hand side of the frame. The free end of the wire is then pulled to break it free from this bond pad, or it is cut.
In a next step, the frame is moved so that the bus bar on the left is located under the tip and the wire is centered between the first and second pad. The wire is then bonded to the left bus bar.
The free end of the wire is then pulled to break it free from the bond and the wire is then bonded to the center of the next available pad on the right hand side of the frame. The free end of the wire is then pulled to break the wire free from the bond, or the wire is cut at this point.
The process is then repeated to produce a parallel grid of uniformly spaced and tensioned wires at a uniform distance apart from each other. By individually fixing the free end of the wire, such as by parallel gap welding it to either the pad or the bus bar on one side of the frame while keeping the wire at a constant tension and then bonding it to the opposite side of the frame, absolute consistency in the tension applied to each wire of the entire grid is assured.
This wiring process can proceed by hand, by using a mechanical stage to accurately and easily position the assembly with respect to the tip of the welder. It can also be a computer controlled process similar to that used in the wire bonding of semiconductor devices into packages.
Fabricating the bus bar and termination pads as a patterned metal film on a ceramic substrate also produces an advantage that prior art techniques do not. In particular, the bus bars and wires form a characteristic impedance that is presented to the electronic circuitry that drives the grid voltage. By keeping the bus bars at a controlled tolerance in terms of their thickness and width on the ceramic substrate, as well as the size of the pads, the characteristic impedance of the wire grid assembly and, in particular, the bus bar itself, can be assured to match that of the driver circuitry. This, in turn, further eliminates another inconsistency with prior art approaches.
The method also allows fabrication of gates with wires several times smaller in diameter than that utilized by other methods.
The square shape of the center hole allows precise alignment of the orientation of the grid wires.
The metalized surfaces of the ceramic reduce the possibility of surface charge build-up during operation, since both the xe2x80x9cfrontxe2x80x9d as well as the xe2x80x9cbackxe2x80x9d are metalized.
The wire can be selected to decrease the thermal coefficient of expansion of the wire relative to the ceramic, for example, using Alloy 46.
A grid constructed according to the invention also lends itself to implementation in quadrupole and higher order multipole structures. For example, two grids may be placed face to facexe2x80x94in this case the spacing between the grids needs to be similar to the spacing between the wires. Nowak and similar prior art approaches that use relatively thick frames do not lend themselves to implementation in such multipole structures.
For example, the bus bars and the wires can be placed symmetrically with respect to a centerline of the support frame, such that by placing a second grid over the first, the bars of the same polarity are opposing each other (to avoid arcing between +V and xe2x88x92V) while wires of opposite polarity were opposing each other so as to produce a quadrupole field. The quadrupole field has a higher deflecting power for the same applied voltage, which reduces energy corruption effects.